Low temperature sub-nanometer periodic stack dielectrics

ABSTRACT

MIM capacitors using low temperature sub-nanometer periodic stack dielectrics (SN-PSD) containing repeating units of alternating high dielectric constant materials sublayer and low leakage dielectric sublayer are provided. Every sublayer has thickness less than 1 nm (sub nanometer). The high dielectric constant materials could be one or more different materials. The low leakage dielectric materials could be one or more different materials. For the SN-PSD containing more than two different materials, those materials are deposited in sequence with the leakage current of the materials from the lowest to the highest and then back to the second-lowest, or with the energy band gap of the materials from the widest to the narrowest and then back to the second widest in each periodic cell. A layer of low leakage current dielectric materials is deposited on and/or under SN-PSD. The dielectric constant of SN-PSD is much larger than that of the component oxides and can be readily deposited at 250° C. using atomic layer deposition (ALD). The ALD deposition cycle could be 20-1000 cycles. The deposition technology is not limited to ALD, could be thermal oxidation, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) and other thermal source assisted deposition.

TECHNICAL FIELD

The present invention relates generally to materials. Specifically, thepresent invention is related to low temperature sub-nanometer periodicstack dielectrics (SN-PSD) with high capacitance density, low leakagecurrent, and the high break down voltage, and related processes.

BACKGROUND

Frequent challenges arise with respect to the dielectric materials. Forinstance, various material properties are desirable, but may not all bepresent in a particular dielectric material. Thus, there is a need forlow temperature sub-nanometer periodic stack dielectrics with highcapacitance density, low leakage current, and the high break downvoltage.

SUMMARY

The present application discloses fabrication of MIM capacitors usingSN-PSD layers composed of TiO₂ and HfO₂ and other materials deposited byALD or other deposition methods. Herein is provided a characterizationof the reliability and demonstration of the superior performance of theSN-PSD films, for example, as compared to single layered dielectricsunder similar capacitance density. The voltage driving breakdown andtime dependent dielectric breakdown (TDDB) tests are used tocharacterize the reliability of SN-PSD MIM devices.

Further disclosed is the use of self-assembled monolayers as cappinglayers, significantly improving the performance of HfO₂/TiO₂ SN-PSDmetal-insulator-metal capacitors. In particular, by examining the effectof controlling the chain length of alkylphosphonic acid molecules, anoctadecylphosphonic acid self-assembled monolayer may be provided withboth desirable suppression of leakage current and minimization ofcapping layer thickness. In various embodiments, a 60% improvement incapacitance density compared to HfO₂ capping layer may arise. Othermaterials of Barium-Strontium Titanate (BST), ZnO₂, and other highdielectric materials are potential of SN-PSD.

DESCRIPTION OF THE FIGURES (BRIEF DESCRIPTION)

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1A depicts a schematic cross-sectional view of a semiconductorstructure according to an embodiment of the process of the invention,after a SiO₂ layer is deposited on top of a Si substrate, in accordancewith various embodiments.

FIG. 1B depicts a schematic cross-sectional view of a semiconductorstructure after a Ti/Pt 30/150 nm bottom contact is deposited usingelectron-beam evaporation, in accordance with various embodiments.

FIG. 1C depicts a schematic cross-sectional view of a semiconductorstructure with a mask to protect the bottom electrode before an SN-PSDdeposition, in accordance with various embodiments.

FIG. 1D depicts a schematic cross-sectional view of a semiconductorstructure after a bottom low leakage current layer, a SN-PSD depositionby ALD, and/or a top low leakage current layer in accordance withvarious embodiments.

FIG. 1E depicts a schematic cross-sectional view of a semiconductorstructure after a shadow mask is applied to define a top electrode area,in accordance with various embodiments.

FIG. 1F depicts a schematic cross-sectional view of a semiconductorstructure after a Pt top electrode is deposited using electron beamevaporation, the shadow mask is stripped and the mask to protect thebottom metal electrode removed, in accordance with various embodiments.

FIG. 2A depicts a schematic cross-sectional view of the SN-PSDdielectric materials with a bottom leakage current cap layer on asubstrate in accordance with various embodiments.

FIG. 2B depicts a schematic cross-sectional view of a stack structure ina periodic cell of the SN-PSD containing two different materials.

FIG. 2C depicts a schematic cross-sectional view of a stack structure ina periodic cell of the SN-PSD containing three different materials.

FIG. 2D depicts a schematic cross-sectional view of a stack structure ina periodic cell of the SN-PSD containing five different materials.

FIG. 3 depicts a comparison of the various capacitor structures in termsof capacitance density and leakage current for SN-PSD capacitors usingTiO₂, Al₂O₃, and HfO₂, in accordance with various embodiments.

FIG. 4 depicts the current-voltage (I-V) characteristic of single layerand SN-PSD capacitors up to breakdown voltage. The inset is a full sweepfrom −3V to +3V, in accordance with various embodiments.

FIG. 5A depicts leakage current density of an Al₂O₃/TiO₂ SN-PSDcapacitor vs. stress time under different voltages, in accordance withvarious embodiments.

FIG. 5B depicts leakage current density of an HfO₂/TiO₂ SN-PSD capacitorwith stress time under different voltages, in accordance with variousembodiments.

FIG. 6 depicts dielectric breakdown lifetime of various SN-PSDcapacitors, in accordance with various embodiments.

FIG. 7A depicts leakage current density J at 100 kHz as a function ofvoltage for SN-PSD capacitors with a capping layer of nothing, BPA SAM,DPA SAM, TDPA SAM, ODPA SAM and 5 nm HfO₂, in accordance with variousembodiments.

FIG. 7B depicts capacitance density at 100 kHz as a function of voltagefor SN-PSD capacitors with a capping layer of nothing, BPA SAM, DPA SAM,TDPA SAM, ODPA SAM and 5 nm HfO₂, in accordance with variousembodiments.

DETAILED DESCRIPTION

This disclosure involves high dielectric constant (high-k) capacitors.More particularly, the present application relates to semiconductorprocesses that optimize high-k capacitor performance.

High dielectric constant materials (high-k) have attracted ongoingresearch interest for their applications in transistors and memories. Tominimize leakage current effect, a thicker capping layer is commonlyused in series with the high-k material to reduce quantum mechanicaltunneling and enhance the device reliability. If high-k is intended fornovel devices on glass and flexible plastics substrate, low temperaturefabrication process is implementable in order to be compatible with thesubstrates. In various instances, 150 nm thick sub-nanometric periodicstacked dielectric (SN-PSD), containing alternating Al₂O₃ and TiO₂sublayers, can exhibit theoretical relative dielectric constant of up to1,000 when the sublayer thickness was less than 0.5 nm. Since therelative dielectric constant of the SN-PSDs is much greater than that ofthe component oxides and can be readily deposited at 250° C. usingatomic layer deposition (ALD), it offers a promising route towardfabrication of high performance metal-insulator-metal (MIM) capacitors.However, even when the expected dielectric constant of standalone SN-PSDcan be great, these films may often be deposited on top of anotherdielectric material (relatively low-k such as SiO₂) to limit the leakagecurrent, which may reduce the overall capacitance in the MIM structureand may make them less practical to be implemented in conventionaltechnologies.

The performance of MIM capacitors containing SN-PSD dielectrics isdescribed using parameters such as capacitance density (C_(den)),leakage current density (J) and reliability. Leakage current is causedin part by the SN-PSD architecture consisting of an interpenetratingnetwork of low and high resistance domains, and as the domain thicknessof a dielectric decreases, quantum tunneling of electrons and henceleakage current becomes increasingly problematic. Past solutionsresolved the leakage current issue by depositing a thick (˜5 nm) cappinglayer of Al₂O₃ on top of the SN-PSD. However, the low relativedielectric constant value of Al₂O₃ (˜9 nm) in series with the SN-PSDlimits the maximum C_(den) of the overall stack to ˜16 fF/μm².

Thus, there may be a desire to improve the performance of SN-PSD MIMcapacitors by increasing capacitance density C_(den) and reliabilitywhile decreasing leakage current J.

The dielectric behavior of MIM capacitors made by SN-PSD with a cappinglayer can be qualitatively understood as a SN-PSD capacitor connectedwith a topping layer capacitor in series. HfO₂ has a dielectric constantof 25-30, while Al₂O₃ has a relatively low dielectric constant of 9-12,therefore, we employed HfO₂ to replace Al₂O₃ in order to further improvethe dielectric performance of SN-PSD devices.

In addition to the dielectric characteristics offered by thesematerials, reliability is a critical factor for applications inelectronic devices. Hf is in the same element group as Ti, which mayreduce the interface defects of HfO₂/TiO₂ due to the similar electronicstructure and crystal configuration of their component oxide, and thenimprove the stability of SN-PSD material. Moreover, HfO₂ has a betterreliability than that of Al₂O₃. So the introduction of HfO₂ may not onlyenhance the performance but also the reliability of SN-PSD devices.

MIM capacitors with two different types of SN-PSD consisting ofalternating subnanometer layers of Al₂O₃/TiO₂ and HfO₂/TiO₂ werefabricated. FIG. 1A, FIG. 1B, FIG. 1C, FIG. 1D, FIG. 1E and FIG. 1Fillustrate the MIM capacitors fabrication process consisting in FIG. 1Aof 300 nm of thermally growth SiO₂ 102 on top of a Si substrate 101.FIG. 1B shows 30 nm of Ti on top of 150 nm of Pt which were evaporatedby e-beam (Temescal 1800) to form a common bottom electrode 103. Thesubstrate was then clipped into multiple pieces of 20×30 mm² area, thenin FIG. 1C a mask 104 was used to protect the bottom contact area priorto SN-PSD 105 deposition by ALD (Cambridge NanoTech Savannah 100 ALD).FIG. 1D shows the SN-PSD 105 was deposited implementing severaldielectric layers configurations and thickness. Al₂O₃/TiO₂ and HfO₂/TiO₂were the two different SN-PSD stack material configurations, for0.7-0.3, 0.5-0.5 and 0.3-0.7 nm were used keeping a total thickness of1.0 nm SN-PSC layer. The final devices were in various instances made upof 50, 75, 100 and 150 layers using 0, 1, 2, 3, 4 and 5 nm of Al₂O₃ orHfO₂ as capping layer to reduce the leakage current for TiO₂/Al₂O₃ andTiO₂/HfO₂, respectively. FIG. 1E shows a shadow mask 106 was attached tothe SN-PSD surface in order to define the top electrodes using Pt (200nm) deposited by e-beam. The top electrode 107 diameters were 100, 200,300 and 500 μm. (F) Then, the masks for the top 106 and bottom 104electrodes were removed for testing.

Since the dielectric constant of SN-PSD is much large than that of thecomponent oxides and can be readily deposited at 250° C. using atomiclayer deposition (ALD), the ALD deposition cycle in various instancesshould be 20-1000 cycles. The deposition technology is not limited toALD, and includes thermal oxidation, chemical vapor deposition (CVD),plasma enhanced chemical vapor deposition (PECVD) and other thermalsource assisted depositions.

The electrical characterization of the MIM capacitors was measured atroom temperature using a Keithley 4200 Semiconductor CharacterizationSystem and Agilent 4284A Precision LCR Meter.

The SN-PSD is not limited to contain two different materials in aperiodic cell. It may contain more than two different materials. Thosematerials are deposited in sequence with the leakage current of thematerials from the lowest to the highest and then back to thesecond-lowest, or with the energy band of the materials from the widestto the narrowest and then back to the second widest in each periodiccell. But the stack structure in a periodic cell is not limited to thesesequences, as long as it can reduce the leakage current of SN-PSD.

FIG. 2A shows a schematic cross-sectional view of the dielectricmaterials with a bottom leakage current cap layer in a SN-PSD basedcapacitor in accordance with various embodiments. Wherein 201 is asubstrate with a layer of buffer material and a layer of bottomelectrode, 202 is a bottom cap layer to reduce the leakage current ofSN-PSD MIM capacitors and 203 is SN-PSD dielectric.

FIG. 2B shows a schematic cross-sectional view of a stack structure in aperiodic cell of the SN-PSD dielectrics containing two differentmaterials. Wherein, 204 is a low leakage current material or a wideenergy band gap material and 205 is a high dielectric constant materialor a narrow band gap material.

FIG. 2C shows a schematic cross-sectional view of a stack structure in aperiodic cell of the SN-PSD containing three different materials.Wherein, 206 is the lowest leakage current material or the widest energyband gap material, 207 is the second lowest leakage current material orthe second narrowest band gap material and 208 is the third lowestleakage current material or the third narrowest band gap material.

FIG. 2D shows a schematic cross-sectional view of a stack structure in aperiodic cell of the SN-PSD containing three different materials.Wherein, 209 is the lowest leakage current material or the widest energyband gap material, 210 is the second lowest leakage current material orthe second narrowest band gap material, 211 is the third lowest leakagecurrent material or the third narrowest band gap material, 212 is theforth lowest leakage current material or the forth narrowest band gapmaterial, 313 is the fifth lowest leakage current material or the fifthnarrowest band gap material.

FIG. 3 illustrates the capacitance density vs. leakage current densityat 1 V bias voltage for SN-PSD with different thickness ratio ofAl₂O₃/TiO₂ and HfO₂/TiO₂, different cycle number of the sublayer (i.e.the total thickness of SN-PSD) and different thickness of capping layerwe fabricated. As can be seen, we can differentiate at least threegroups with specific capacitance and leakage current characteristics.Single layer capacitors 301 refer to structures consisting indielectrics made out of single materials such as HfO₂, Al₂O₃ and TiO₂(squares 301). At 5 nm dielectric thicknesses, this group offerscapacitance densities above 20 fF/μm² at the cost of higher leakagecurrents (above 1×10⁻³ A/cm²) compared to capacitors made with SN-PSD.Although capacitance density is high, the implementation of thesedevices as capacitive element in devices and circuits may in variousinstances be associated with a high leakage current that generates largepower consumption and/or limitations to device reliability. In variousinstances, a solution to decrease the leakage current is by increasingthe dielectric thickness that also results in a decrease in capacitancedensity as shown for devices with 20 nm thickness in FIG. 3.

The second group of devices 302 consists of MIM structures made withAl₂O₃/TiO₂ SN-PSDs. When capping layer is at least 5 nm Al₂O₃, thesecapacitors present leakage current densities below 1×10⁻⁵ A/cm² andcapacitance densities of 13.2 fF/μm² and 17.2 fF/μm² for devices with150 nm and 50 nm of SN-PSD Al₂O₃/TiO₂ ratio of 0.3 nm/0.7 nm (150ATO375and 50ATO375), respectively. As can be seen from FIG. 3, capacitancedensity could be further increased up to 22.7 fF/μm² by decreasing thethickness of the Al₂O₃ capping layer to 4 nm (150ATO374), however thisapproach also increases leakage current in more than two orders ofmagnitude compared to the use of 5 nm Al₂O₃ capping layer.

The third group 303 consists of devices made with 100 nm SN-PSD withHfO₂/TiO₂ ratio of 0.7 nm/0.3 nm with 5 nm HfO₂ top capping layer(100HTO735) and 50 nm SN-PSD with HfO₂/TiO₂ ratio of 0.7 nm/0.3 nmwithout capping layer (50HTO73). FIG. 3 clearly shows that deviceswithout capping layer at all yield higher leakage current performances,whereas 100HTO735 offers capacitance and current leakage densitiesfollowing the same trend as devices with Al₂O₃/TiO₂ SN-PSDs indicatingthat both electrical characteristics are dictated not only by thethickness of the SN-PSD but also the thickness of the top capping layer.

A full reliability study was performed to the SN-PSD and single layerdevices presenting similar capacitance density and current leakagecurrent (enclosed in the circle 304 in FIG. 3). The initial dielectriccharacteristics of the four samples for reliability test are summarizedin Table 1.

VG = 1 V Equivalent Silicon Capacitance Current Sample ThicknessEffective Oxide Thickness Density Density Name Description (nm) K (nm)(fF/um²) (A/cm²) 150ATO375 Al₂O₃ 155 257.00 2.29 15.54 6.38E−08 0.3 nm,TiO₂ 0.7 nm, 150 cycles, Capping layer: Al₂O₃ 5 nm 100HTO735 HfO₂ 0.7nm, 105 241.94 2.43 14.63 1.08E−06 TiO₂ 0.3 nm, 100 cycles, Cappinglayer: HfO₂ 5 nm HfO2-9 nm 9 nm HfO₂ 9 14.83 2.30 14.60 1.85E−07 singlelayer capacitor, performed reliability test Al2O3-6 nm 6 nm Al₂O₃ 6 7.672.97 11.38 1.64E−07 single layer capacitor, performed reliability test

FIG. 4 illustrates the I-V characteristics of four devices with similarcapacitance densities including 150ATO375 and 100HTO735 SN-PSD devicesand single layer capacitors. As voltage keeps increasing, the devicescontaining single layer Al₂O₃ 401 and HfO₂ 402 start the breakdownprocess at 3 V and 4.5 V, respectively, while the ones with theAl₂O₃/TiO₂ SN-PSD 403 (150ATO375) and HfO₂/TiO₂ SN-PSD 404 (100HTO735)break down until about 6.1 and 6.9V, respectively. The breakdown isdefined when the leakage current increases ten times the value of theprevious test step or the leakage current increasing to 1 mA for thedevice with the diameter of 100 um (12.74 A/cm²). The reason for thiscriterion may be given by the fact that original performance on eachdevice was almost 100% recoverable after releasing the voltage stress ifthe leakage current was kept below 1 mA for less than 100 s.

The SN-PSD capacitors 403 and 404 exhibited higher breakdown voltagethan the traditional single layered HfO₂ 402 and Al₂O₃ 401 capacitors.This may in various instances be associated with a very large dielectricconstant such as may be caused by Maxell-Wagner relaxation. In variousinstances, the sub-nanometer periodic stack structure can be physicallythick and still deliver high capacitance densities. The actual electricfield may be reduced significantly under the same bias voltage. Thebreakdown voltage of SN-PSD is improved.

Considering only the SN-PSD capacitors, the breakdown voltage of thedevice with HfO₂/TiO₂ 404 was higher than the one with Al₂O₃/TiO₂ 404.This may be associated in various embodiments with the thicker insulatorin each periodic sublayer of HfO₂/TiO₂ based SN-PSD (HfO₂—0.7 nm) thanthat of Al₂O₃/TiO₂ based SN-PSD (Al₂O₃—0.3 nm) with the similarcapacitance density, as well as the relatively low leakage current andoptimized stability of HfO₂ and even the interface states of HfO₂/TiO₂.

Inset in FIG. 4 illustrates the full sweep from −3 V to +3 V for thefour devices where it can be noticed some level of asymmetry in thecurrent values which could be the result of the capping layer. The 5 nmcap layer was deposited on the top of SN-PSD. TiO₂ is a typical electrontransport semiconductor, while HfO₂ and Al₂O₃ are typical insulators. Itis worth pointing out that the bottom electrode of the devices wasbiased at 0 V. The lower leakage current at positive bias may in variousinstances be a result of a potential barrier formed by the top cappinglayer able to block the electrons flowing from the SN-PSD layers. Italso can be seen from the inset in FIG. 4 that the leakage current atlow voltages increases more rapidly for sample 100HTO735 412 compared to150ATO375 411 and single layer capacitors 413, 414 in about two ordersof magnitude. This may in various instances be at least partially due tothe different dielectric properties of HfO₂ and Al₂O₃. HfO₂ has arelatively narrow band gap of 5.5 or 6 eV, while Al₂O₃ has a relativelywide band gap of 8.8 or 8.9 eV. Therefore, there may be a relativelysmall band barrier at the interface between HfO₂/TiO₂ than that ofAl₂O₃/TiO₂ to block the flow of electrons, which results in therelatively higher leakage currents for HfO₂/TiO₂ SN-PSD MIM device.

Time dependent dielectric breakdown (TDDB) characterization of thecapacitors enclosed in the circle 304 in FIG. 3 may be performed underdifferent stress voltages. Our results may show that the hard breakdownof the Al₂O₃/TiO₂ SN-PSD capacitors occurred at much higher stressvoltage compared to the hard breakdown for 6 nm Al₂O₃ capacitors thatoccurred immediately after applying stress voltages of 2.5V, 2.75V, 3Vand 3.25V. The hard breakdown of 9 nm HfO₂ capacitor occurred at theapplied stress voltages of 4.25V and 4.5V. This means the SN-PSDcapacitor has much stronger tolerance to the stress voltage than thesingle layer Al₂O₃ or HfO₂ capacitor with the similar capacitancedensity. Single layered devices (HfO₂ and Al₂O₃) in various instancespresent a monotonic decrease of leakage current with the stress timethat has been attributed to a charge trapping mechanism. As charge istrapped inside the capacitor dielectric, it creates an additionalelectric field which opposes more charge to enter the insulator and,thus, the current decreases progressively.

FIG. 5A shows the current vs. time curves of the Al₂O₃/TiO₂ SN-PSDcapacitors under different stress conditions. Similar accelerateddegradation to the single layer of Al₂O₃ or HfO₂ high k capacitors whenthe stress voltage is as low as 2.5V can be seen, i.e. the leakagecurrent decreases monotonically with the stress time 511. When thestress voltage was biased at 3V 512, the leakage current of theAl₂O₃/TiO₂ SN-PSD capacitor was observed to first decrease and thenslightly increase with the stress time (FIG. 5A). However, the leakagecurrent of the Al₂O₃/TiO₂ SN-PSD capacitor increased gradually with thestress time when the stress voltage was higher than 4V 513, 514, quitedifferent from the single layer Al₂O₃ or HfO₂ capacitors.

HfO₂/TiO₂ SN-PSD capacitors experienced similar accelerated degradationtrend compared to Al₂O₃/TiO₂ SN-PSD capacitors, as shown in FIG. 5B. Theleakage current under 2.5V stress 521 almost did not change within 1000S stress time. The leakage current increased gradually with the stresstime when the stress voltage was higher than 4V 523, 524.

For the SN-PSD capacitor, the mismatch of the lattice structure of Al₂O₃or HfO₂ and TiO₂ may create large amounts of interface electron statesadditional to the electron trap states inside the sub-nanometer thinfilms. All those electron states may trap electrons which were injectedfrom the low potential biased electrode. We believe those electron trapstates led to the difference of the change trends of the leakage currentinside the SN-PSD capacitor with the stress time from the single layerinsulator capacitor. When the stress voltage is as low as 2.5V in FIG.5A 511 or 2V in FIG. 5B 521, the electrons initially filled the traps atlow energy levels. Like the single layer component oxide capacitor, itmay create an additional electric potential to oppose more carriers toenter the SN-PSD capacitor. Thus, the current decreased with the stresstime.

At the beginning time of the voltage stress, more electrons are quicklytrapped than de-trapped and, thus, the leakage current decreasesquickly. As the stress voltage increases, more electrons fill more trapstates even at higher energy level while more electrons likely de-trapinto the conduction band of the SN-PSD dielectric. With further increaseof the stress time, more electrons will likely be de-trapped and theleakage current will increase slowly.

In our TDDB tests the Keithley 4200 SCS was set up in quiet mode, thismeans, it takes about ten seconds to sample the first current value. Wethink the large stress voltage (i.e. large applied electric field)rapidly fills the electron trap states and the Keithley 4200 SCS is notable to read the decrease of current at the very beginning of thevoltage stress. Meanwhile, the large stress voltage may increase theprobability of the electron detrapping from the filled electron trapstates. Therefore, it was observed that the leakage current increasedmonotonically with the stress time in the cases of large voltage stress(like 5V in FIG. 5A 514, and 5V in FIG. 5B 524).

I-V and C-V curves were collected before and after the SN-PSD capacitorswere stressed at different voltages for ˜1000 s. The C-V and I-V curvesof Al₂O₃/TiO₂ SN-PSD capacitors after stressed for ˜1000 s is almost thesame as the initial ones (before the voltage stress) when the stressvoltage is lower than or equal to 4 V. This implies that the TDDBdegradation under stress voltages lower than 4V is caused by occupationstatus of the electron trap states. As the stress bias is removed, theelectron trap states reinstate to the initial status and, thus, thedielectric properties of the SN-PSD capacitor are 100% recovered.

FIG. 5A also illustrates that the current of Al₂O₃/TiO₂ SN-PSDcapacitors increased up to five orders of magnitude when the stressvoltage was higher than 5 V 514. The voltage stress may change theoccupation status of the electron states inside the sub-films and at theinterfaces between two adjacent sub-films. Higher stress voltage mayresult in more filled electron trap states and then more electrons jumpout from the trap states to enter the conduction band. However, I-V andC-V characteristics were in various embodiments not reinstated to theoriginal behavior after the stress voltage was resealed. Therefore,there may be further mechanisms causing the degradation of thedielectric performance of the SN-PSD capacitors. For instance, invarious instances the large voltage stress may induce the change of theelectron trap states, migration of oxygen or the other ions, thedegradation of the dielectric performance at the sub layer level, thedegradation of Maxwell-Wagner type dielectric relaxation or somediffusion of the metal electrode into the dielectric layers.

After stressed at 6.5V (higher than the breakdown voltage of 6.1V) for1000 S, no C-V characteristics were measured. The dielectric propertiesof the device were unrecoverable (hard breakdown). The breakdown was notobserved to be recovered in any degree by removing the stress voltage.It is important to note that devices were re-measured 5 months after thebreakdown without successfully recovering the original behavior.

The recorded current of the hard broken-down Al₂O₃/TiO₂ SN-PSDcapacitors was linear within the range of [−105 mA, 105 mA], which isthe highest compliance of a Keithley 4200 SCS system used. But theleakage current of the broken down SN-PSD capacitor was lower than thebroken down single layer HfO₂ or Al₂O₃ capacitor. I-V curves illustratethere is a conductive path created with the breakdown in the dielectriclayer, which may be caused by some electrode metal diffusing into orTiO₂ into the Al₂O₃ sublayers. The low leakage current in the brokendown SN-PSD capacitor shows that there may be no metal path formedbetween top and bottom electrode. This may be attributed to the thickSN-PSD dielectric layers and low conductivity of TiO₂.

The C-V and I-V curves of HfO₂/TiO₂ SN-PSD capacitors before and afterstress for all bias voltages for ˜1000 s are almost the same. Nounrecoverable damage was observed after the stress was released. So onemay appreciate that the TDDB degradation of HfO₂/TiO₂ SN-PSD under thestress conditions may be caused by electron trapping and de-trapping.

The I-V and C-V measurement at initial time and after the breakdown testshow that soft breakdowns occurred in the Al₂O₃/TiO₂ SN-PSD capacitorswhen was performed the voltage driving breakdown test or the TDDB testunder the voltage stress lower than 4V. The performance of Al₂O₃/TiO₂SN-PSD capacitor was observed to be recovered immediately after thesweeping voltage or the voltage stress was removed. The soft breakdownwas observed in HfO2/TiO2 SN-PSD capacitors under all the voltage stressfor 1000 s. The soft breakdown was also observed occurring in singlelayer HfO₂ capacitors under voltage of 4V as well, but not observedoccurring in single layer Al₂O₃ capacitor.

I-V measurement at initial time and after breakdown under voltage stressof 5V also illustrates the leakage current in the broken-down Al₂O₃/TiO₂SN-PSD decreased by 2 orders of magnitude after it was put in the air atroom temperature for ˜5 months. The performance degradation of thebroken-down Al₂O₃/TiO₂ SN-PSD capacitor with a cap layer was partlyrecovered. It means the mechanisms of stress induced degradation mayinclude recoverable and un-recoverable damages.

The performance of HfO₂/TiO₂ SN-PSD capacitor was immediately recoveredafter each voltage stress for 1000 s when the compliance of Keithley4200 was set at 1 mA. But the performance degradation was observed whenthe compliance of Keithley 4200 was set at 100 mA. It demonstrates theHfO₂/TiO₂ SN-PSD capacitor is in various embodiments more reliable thanthe Al₂O₃/TiO₂ SN-PSD and single layer capacitors.

The TDDB lifetime extrapolation in FIG. 6 illustrates that dielectricbreakdown lifetime in various embodiments would exceed 10 years underthe stress voltage of 4.0V for HfO₂/TiO₂ SN-PSD capacitors, 2.3V forAl₂O₃/TiO₂ SN-PSD capacitors, and 2.1V for ALD HfO₂ capacitor. Theequivalent electric field normalized by their equivalent silicon oxidethickness is 16.5 Mv/cm for HfO₂/TiO₂ SN-PSD capacitors 601, 10.0 Mv/cmfor Al₂O₃/TiO₂ SN-PSD capacitors 602, and 9.1 Mv/cm for ALD HfO₂capacitor 603. But the dielectric breakdown lifetime of ALD Al₂O₃capacitor may be extrapolated never longer than 10 years 601, no matterhow low the stress voltage is. In other words, the dielectric breakdownlifetime of SN-PSD MIM Capacitor may be longer than the HfO₂ capacitoror Al₂O₃ capacitor under the same stress voltage. Also, HfO₂/TiO₂ SN-PSDcapacitor exhibited much longer lifetime than the AL₂O₃/TiO₂ SN-PSDcapacitor.

To further enhance capacitance density and leakage performance, otherembodiments are contemplated.

High-k dielectric capacitors utilizing SN-PSD are not limited to twosublayers. In another embodiment, three layers of different materialswith HfOx/ZrOx/TiO₂ SN-PSD dielectric material, consisting of 150-1500nm alternating layers with top cap layer of HfOx on both sides.

In another embodiment, three layers of different materials withAlxOy/HfOx/TiO₂ SN-PSD dielectric material are used, consisting of150-1500 nm alternating layers with top cap layer of AlxOy on bothsides.

The high dielectric constant materials used in sublayers further includeAl_(x)O_(y), HfOx, TiO_(x), ZrOx ZnO_(x), SiO₂, SiNx, and BaSrxTiy.Since the dielectric constant of SN-PSD may be much large than that ofthe component oxides and can be readily deposited at 250° C. usingatomic layer deposition (ALD), the ALD deposition cycle may be 20-1000cycles. The deposition technology is not limited to ALD, but includethermal oxidation, chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD) and other thermal source assisteddeposition techniques.

Optimization of an SN-PSD MIM capacitor may include a capping layer,preferably of self-assembled monolayers (SAMs). Self-assembledmonolayers (SAMs) of organic molecules represent a versatile techniqueto modify the properties of metal and metal oxide surfaces. Inparticular, alkane-terminated phosphonic acids binds strongly to avariety of metal and metal oxides and the resulting SAMs exhibitexcellent performance as gate dielectric in organic thin filmtransistors, A octadecylphosphonic acid (ODPA) SAM formed on top of athin AlO_(x) film decreased J by three orders of magnitude. In variousinstances the effect of alkylphophonic acid SAM capping layers on theperformance of SN-PSD MIM capacitors containing TiO₂ and HfO₂ sublayersare disclosed. In particular, we show that varying the alkyl chainlength significantly affects J and breakdown voltage (V_(bd)) of thesecapacitors. Finally, we demonstrate that through the effectivesuppression of J, a SN-PSD MIM capacitor using ODPA SAM capping layerexhibits a 60% improvement in C_(den) compared to control devices usinga HfO₂ capping layer.

ODPA, tetradecylphosphonic acid (TDPA), decylphosphonic acid (DPA),butylphosphonic acid (BPA), and anhydrous isopropanol were purchasedfrom Sigma-Aldrich and were used as received. SN-PSD capacitors werefabricated between Pt electrodes. First, 10 nm Ti/100 nm Pt were e-beamevaporated on Si wafer. HfO₂ and TiO₂ were deposited using atomic layerdeposition by reaction of HfCl₄ and TiCl₄ with H₂O at 250 C, underreaction conditions of, and the film thickness was monitored usingquartz crystal microbalance. For the SN-PSD, 100 cycles of 0.5 nmHfO₂/0.5 nm TiO₂ were deposited, and a final layer of 0.5 nm HfO₂ wasdeposited, yielding a total SN-PSD thickness of ˜100 nm. TheSN-PSD-coated wafer was then cut into˜1.5 cm×2.5 cm pieces. SAM cappinglayers were then deposited on SN-PSDs. 7 mL solution of 5 mM ODPA, TDPA,DPA, or BPA in anhydrous isopropanol was prepared in a 50 mL disposablepolypropylene centrifuge tube (Corning) in N₂ atmosphere. A piece ofSN-PSD sample was placed inside, and the centrifuge tube was capped andtilted so that the SN-PSD was face down and submerged in solution atroom temperature for 16 hours. The sample was then removed fromsolution, rinsed with copious amount of anhydrous isopropanol, driedwith N₂ gun, and placed on a hot plate at 60 C for 10 min in N₂atmosphere. The capacitors were then defined by e-beam evaporation of Ptthrough a shadow mask with 100 μm diameter circular openings. Twoadditional control samples were fabricated, one containing no cappinglayer, and another containing 5 nm HfO₂ on SN-PSD as capping layer. ACcapacitance and current measurements between 100 Hz and 1 MHz as afunction of DC voltage between −3.5 V and 3.5 V and other conditionswere performed in air using a LCR meter, with the top electrode as theworking electrode. At least 3 capacitors were measured per sample forstatistics. For FTIR characterization of SAMs, 5 nm HfO₂ was depositedon Si/Ti/Pt by ALD, and the SAMs were deposited using the proceduredescribed above. FTIR absorbance spectra of SAMs were collected from400-4000 cm⁻¹ in a N₂-purged Therno Nicolet is 50 spectrometer. Thespectrometer is equipped with a home-built grazing incidence reflectionaccessory employing gold coated mirrors to direct the beam at 80 degreeswith respect to the substrate normal. A polarizer was used to selectonly the p-polarized component of the beam. The spectral resolution was4 cm⁻¹, and 3 independent measurements of 512 scans were averaged foreach spectrum. The spectra were referenced to measurements of a cleansubstrate with no SAM attached.

The introduction of SAM capping layers provides tunable suppression of Jand improvement in C_(den) for the SN-PSD MIM capacitors, consistentwith the length of the alkyl terminal group. J-V curve of the SN-PSDcapacitor without capping layer (FIG. 7A, 710) shows an asymmetricresponse, with J at forward bias (positive bias for top electrode)exhibiting higher values than J at reverse bias (negative bias for topelectrode). This may in various instances be caused by the fact that theinterface between SN-PSD and top Pt is buried and therefore is notexposed to air, leading to a work function farther away from the midgapof HfO₂ and hence greater current injection. Treatment with BPA SAM alsocauses J to increase at all V values (FIG. 7A, 711), indicating that theshort molecule exhibits a reduced effectiveness at suppression ofleakage current. Increasing alkyl chain length with DPA (FIG. 7A, 712)and TDPA (FIG. 7A, 7B) results in J that decreases with increasing chainlength, but the value is still higher than the control without cappinglayer. In contrast, the sample with ODPA SAM (FIG. 7A, 714) exhibitedlower J than the control, suggesting that the SAM is finallycontributing to the suppression of leakage current. Finally, we willnote that 5 nm HfO₂ capping layer yields the lowest J values (FIG. 7A,715). The capping layers also significantly influenced C_(den) in twoaspects. First, the expected voltage-independent C_(den) over differentvoltage ranges depending on the capping layer (FIG. 7B). Indeed, C_(den)values become unphysical when J>10⁻² A/cm². Thus, if we define breakdownvoltage V_(bd) as the voltage at which this transition occur for bothforward and reverse bias (Table 2), we see that V_(bd) values increaseswith increasing alkyl chain length in the SAM, but does not exceed thecontrol without capping layer until ODPA (FIG. 7B 724) was used, and isnot as wide as the control with 5 nm HfO₂. At the same time, the cappinglayer negatively impacts C_(den), as expected from geometricconsiderations. The inclusion of an ODPA SAM capping layer 724 decreasedC_(den) at 0V from 27.8±2.6 fF/μm² to 22.0±2.5 fF/μm², which is still60% higher than a capping layer of 5 nm HfO₂ 725 at 13.7±0.7 fF/μm²(Table 2).

TABLE 2 C_(den) at 0 V and V_(bd) at forward and reverse bias for SN-PSDcapacitors with different capping layers. Capping Layer C_(den) 0 V[fF/μm²] V_(bd) forward [V] V_(bd) reverse [V] Nothing 27.8 ± 2.6 0.9 ±0.2 −1.5 ± 0.1 BPA 25.8 ± 0.8 0.2 ± 0.1 −0.2 ± 0.1 DPA 28.0 ± 1.0 0.4 ±0.1 −0.3 ± 0.1 TDPA 18.9 ± 1.1 0.4 ± 0.0 −0.3 ± 0.0 ODPA 22.0 ± 2.5 0.9± 0.1 −3.0 ± 0.5 5 nm HfO₂ 13.7 ± 0.7 >3.0 >3.0

Given the organic nature of the SAMs, environmental and thermalstability of the SN-PSD capacitors represent a reasonable concern. Toevaluate these properties, one may expose the devices to RT air at 30%relative humidity for 20 days, and then measure theircapacitance-voltage and current-voltage response. In variousembodiments, the aged devices displayed unchanged J and C_(den) curvescompared to as-made samples. The devices, upon heating, may atsuccessively higher temperatures in air for 30 min, and exhibit acharacterized response wherein after 100 C heating, the J and C_(den)curves remain unchanged, and the trend continues up to 200 C.

Capping layers consisting of self-assembled monolayers ofalkylphosphonic acids may further significantly improve the performanceof nanolaminate metal-insulator-metal capacitors. The alkylphosphonicacid forms bidentate binding to HfO₂, and the areal density of the SAMon HfO₂ appears independent of the alkyl chain length.Octadecylphosphonic acid SAM capping layer resulted in good suppressionof leakage current and thus a 60% increase in capacitance densitycompared to a 5 nm HfO₂ capping layer. The SAM also exhibits goodenvironmental and thermal stability up to 200 C. Thus, SAM represents aversatile and effective approach to optimize the response ofhigh-performance dielectric capacitors.

One may appreciate that in view of the disclosure herein, variousaspects may be provided in numbered sections below. The followingparagraphs summarize various such aspects consistent with the disclosureherein above.

Numbered Item 1: For example, in various embodiments, there is asub-nanometric periodic stacked dielectrics (SN-PSD) containingalternating high dielectric constant materials sublayer and low leakagedielectric sublayer. Top and bottom layers are low leakage currentdialectic materials. Every sublayer has thickness less than 1 nm. Thehigh dielectric constant materials could be one or more differentmaterials. The low leakage dielectric materials could be one or moredifferent materials. Since the dielectric constant of SN-PSD is muchlarge than that of the component oxides and can be readily deposited at250° C. using atomic layer deposition (ALD), the ALD deposition cyclecould be 20-1000 cycles. The deposition technology is not limited toALD, could be thermal oxidation, chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD) and other thermal sourceassisted deposition.Numbered Item 2: Compared with single layer dielectric, SN-PSD haschanged the breakdown mechanism from hard breakdown to soft breakdown.The breakdown dielectric could be recovered after few days.Numbered Item 3: The aspects of Numbered Item 2 above, further includingwherein the sublayer could be Al_(x)O_(y), HfOx, TiO_(x), ZrO_(x)ZnO_(x), SiO₂, SiNx, BaSrxTiy or other dielectric materials.Numbered Item 3: An Al₂O₃/TiO₂ SN-PSD dielectric material may beprovided, consisting of 150-1500 nm alternating layers of 0.3 nm Al₂O₃and 0.7 nm TiO₂ with at least 5 nm Al₂O₃ top cap layer only, exhibitedthe capacitance density of 15.54 fF/um² and the leakage current of6.38E−8 A/cm² at the bias voltage of 1V. The non-uniformity ofcapacitors is about 5%. (Flow rate: 20 sccm of N2 purging the chamberduring deposition, using 0.015 sec. pulse of Al2O3, 0.1 sec. of pulse Tiand 0.4 sec. pulse of Al.) (Pressure: Base pressure was 0.4 Torr andincrease up to 0.55 Torr in during each pulse.) (Deposition time: For100 nm of SN-PSD with 5 nm of cap layer may be 5 hrs. and 40 min.approximately)Numbered Item 4: The aspects of Numbered Item 2 above, further includingwherein there may be capping layers on both sides of dielectric.Numbered Item 5: Also provided is a HfO₂/TiO₂ SN-PSD MIM dialecticmaterial consisting of thickness of 150 nm-1500 nm, alternating layersof 0.5 nm HfO₂ and 0.5 nm TiO₂ with at least 5 nm HfO₂ top cap layeronly, exhibited the capacitance density of 30.26 fF/um² and the leakagecurrent of 2.1E−05 A/cm² at the bias voltage of 1 V. The non-uniformityof capacitors may be about 5%. (Flow rate: 20 sccm of N2 purging thechamber during deposition, using 0.015 sec. pulse of H2O, 0.1 sec. ofpulse Ti and 0.4 sec. pulse of Hf) (Pressure: Base pressure was 0.4 Torrand increase up to 0.55 Torr in during each pulse.) (Deposition time:For 100 nm of SN-PSD with 5 nm of cap layer may be 5 hrs. and 40 min.approximately)The TDDB lifetime extrapolation indicates that the dielectric breakdownlifetime may exceed 10 years under the stress voltage of 4.0V(equivalent electric field of 16.5 Mv/cm) for HfO₂/TiO₂ SN-PSDcapacitors, 2.3V (equivalent electric field of 10.0 Mv/cm) forAl₂O₃/TiO₂ SN-PSD capacitors, 2.1V (equivalent electric field of 9.1Mv/cm) for ALD HfO₂ capacitor. So, the dielectric breakdown lifetime ofSN-PSD MIM Capacitor is obviously longer than the HfO₂ capacitor orAL₂O₃ capacitor under the same stress voltage. Soft break down wasobserved on SN-PSD MIM devices. Compared to AL₂O₃/TiO₂ SN-PSD capacitor,the newly developed HfO₂/TiO₂ SN-PSD capacitor not only behaved twicecapacitance density, also showed much longer life time.Numbered Item 6: The aspects of Numbered Item 5 above, further includingwherein there may be capping layers on both sides of dielectric.Numbered Item 7: There may be provided, three layers of differentmaterials with HfOx/ZrOx/TiO₂ SN-PSD dielectric material, consisting of150-1500 nm alternating layers with top cap layer of HfOx on both sides.This structure is expected to further improve the capacitance densityand leakage performance.Numbered Item 8: There may be provided, three layers of differentmaterials with AlxOy/HfOx/TiO₂ SN-PSD dielectric material, consisting of150-1500 nm alternating layers with top cap layer of AlxOy on bothsides. This structure is expected to further improve the capacitancedensity and leakage performance.Numbered Item 9: Disclosed herein is the use of self-assembledmonolayers (SAM) as capping layers can significantly improve theperformance of HfO₂/TiO₂ SN-PSD metal-insulator-metal capacitors. Inparticular, by examining the effect of controlling the chain length ofalkylphosphonic acid molecules, we find that an octadecylphosphonic acidself-assembled monolayer offer a good compromise between suppression ofleakage current and minimization of capping layer thickness, resultingin a 60% improvement in capacitance density compared to HfO₂ cappinglayer.Numbered Item 10: The aspects of Numbered Items 1-9 above, furthercomprising high dielectric materials with SAM layers.

What is claimed:
 1. A high dielectric constant thin film manufacturedusing low temperature sub nanometer periodic stack dielectrics (SN-PSD)comprising: a plurality of sublayers, wherein the plurality of sublayersconsecutively alternate between a low leakage dielectric sublayer and ahigh dielectric constant sublayer, wherein both the low leakagedielectric sublayer and the high dielectric sub layer have a thicknessof less than 1 nanometer; wherein the plurality of sublayers are stackedin order according to a value of a leakage current of each of differentmaterials of the plurality of sublayers, from a lowest value to ahighest value to a second-lowest value.
 2. The high dielectric constantthin film of claim 1, further comprising a first capping layerpositioned on a top of the plurality of sublayers and a second cappinglayer positioned on a bottom of the plurality of sublayers, the firstcapping layer and the second capping layer configured to limit currentleakage.
 3. The high dielectric constant thin film of claim 2, whereinthe first capping layer and the second capping layer comprise any of aplurality of inorganic current blocking materials or organic currentblocking materials.
 4. The high dielectric constant thin film of claim3, wherein the plurality of inorganic current blocking materials ororganic current blocking materials comprise any of: a pure form of anyof a plurality of materials; a doped form of any of the plurality ofmaterials; a self-assembled monolayer made from octadecylphosphonicacids; a base alloy of the plurality of materials; or a mixture of oneor more of the plurality of materials; the plurality of materialscomprising HfOx, HfxSiOy, ZrOx, ZrxSiOy, LaxGdOy, SiOx, SiNx, SiOxNy,and TaOx.
 5. A high dielectric constant thin film manufactured using lowtemperature sub nanometer periodic stack dielectrics (SN-PSD)comprising: a plurality of sublayers, wherein the plurality of sublayersconsecutively alternate between a low leakage dielectric sublayer and ahigh dielectric constant sublayer, wherein both the low leakagedielectric sublayer and the high dielectric sub layer have a thicknessof less than 1 nanometer; wherein the plurality of sublayers are stackedin order according to a value of an energy band gap from widest tonarrowest to second widest.
 6. The high dielectric constant thin film ofclaim 5, further comprising a first capping layer positioned on a top ofthe plurality of sublayers and a second capping layer positioned on abottom of the plurality of sublayers, the first capping layer and thesecond capping layer configured to limit current leakage.
 7. The highdielectric constant thin film of claim 6, wherein the first cappinglayer and the second capping layer comprise any of a plurality ofinorganic current blocking materials or organic current blockingmaterials.
 8. The high dielectric constant thin film of claim 7, whereinthe plurality of inorganic current blocking materials or organic currentblocking materials comprise any of: a pure form of any of a plurality ofmaterials; a doped form of any of the plurality of materials; aself-assembled monolayer made from octadecylphosphonic acids; a basealloy of the plurality of materials; or a mixture of one or more of theplurality of materials; the plurality of materials comprising HfOx,HfxSiOy, ZrOx, ZrxSiOy, LaxGdOy, SiOx, SiNx, SiOxNy, and TaOx.